1. Field of the Invention
The present invention relates generally to aircraft antennas.
2. Description of the Related Art
The Distance Measuring Equipment (DME) system is configured to be especially useful in aircraft navigation. The DME system operates in various channels across the frequency band between 900 and 1200 MHz and computes distance from a ground-based receiver by measuring the time signal pulses take for a transmitted pulse to transit between an aircraft and the receiver and then return to the aircraft. In the DME system, aircraft ground speed can also be found by averaging the change in distance.
In contrast, Transponders are the airborne component of the Air Traffic Control Radar Beacon System and are especially useful in aircraft identification and navigation. In this system, aircraft are interrogated with pulses at a frequency of 1030 MHz. When an aircraft receives the interrogation, its transponder sends a reply at a frequency of 1090 MHz. The interrogator's processor then decodes the reply, identifies the aircraft and determines the range via the delay between interrogation and reply. Aircraft azimuth is determined from antenna pointing directions.
More recently, the Traffic Alert Collision Avoidance System (TCAS) has been developed in response to a history of disastrous mid-air accidents. It is a predictive warning system that is included within a broader airborne collision avoidance system (ACAS) to reduce aircraft collisions. It employs surveillance radar transponders with signal interrogations to survey and determine the presence of airspace conflicts. In particular, the system constantly evaluates a specific volume of airspace and the geometry around it to resolve conflicts within the airspace.
In at least one mode of TCAS, the interrogations cooperate with any aircraft that is equipped with an appropriate transponder. Omni-directional signals (generally known as squitters) are radiated once per second to announce an aircraft's presence to other like-equipped aircraft. Following receipt of a squitter, TCAS sends an interrogation at a selected frequency (e.g., 1030 MHz) to the address contained in a received message. Intruder range is determined by the time delay between interrogation and the reply sequence at a selected frequency (e.g., 1090 MHz) and this process may be effectively managed with a whisper-shout routine.
In the whisper-shout routine, signal transmission begins with a first pair of low-power interrogation pulses—a whisper that nearby aircraft can respond to. After a short delay, a second pair of interrogation pulses is radiated at a higher power. This second pair, however, is preceded by a pulse at the lower power of the first pair. Any transponders that replied to the first pair will not reply to the shout of the second pair. This process can be continued with progressively higher-power pulses to thereby solicit responses while reducing the signal traffic flow.
If this TCAS process indicates an imminent collision, the interrogating aircraft is advised to take action vertically to avoid the developing threat. Vertical flight-path changes have been found to be the quickest resolution to a possible conflict. This action must be accomplished prior to the “closest point of approach” (CPA) which is the point ahead that the aircraft's processor predicts will be an area of conflict with an intruder aircraft. Rather than distance, TCAS processors generally concentrate on determining the time in seconds to the CPA and the horizontal miss distance to the CPA. In the presence of high closure rates (e.g., 1200 knots) and high vertical rates (e.g., 10,000 feet per minute) time is obviously critical. Even so, some TCAS interrogation modes can simultaneously track numerous aircraft (e.g., 30) in a wide coverage range (e.g., 30 nautical miles).
System accuracy and sensitivity are highly reliant on the ability of TCAS antennas to form a steerable beam of enhanced antenna gain. Preferably, an electronically-controlled solid angle is created by differential phase control of the TCAS antennas. With precise phase control, a TCAS processor is then able to place the antenna beam in any of multiple locations about an aircraft. Because phase control is critical to system accuracy, it is important to control any variation in phase due to changes in aircraft antennas and interconnecting cables. Therefore, internal phase calibration is preferably provided to remove errors due to physical changes in the system (e.g., in cables and production electrical disconnects).
Beam steering can be used to transmit interrogations to selected solid-angle segments of the antenna array while transmission to non-selected areas is suppressed. The antenna angle of arrival can be further resolved by directional interrogations in the remaining zones around the aircraft. A TCAS processor can control signal phasing of all elements in the antenna to thereby direct radiation patterns around the aircraft.
From the above descriptions, it is apparent that the combined operations of the DME, Transponder and TCAS systems places demanding performance requirements on antenna structures.